Phylum Aschelminthes

Six different classes, dominated by the Class Nematoda, represent Aschelminthes as a phylum. The nematodes (Nema, thread) or roundworms are ubiquitous small worms that dwarf in numbers all other groups of multicellular animals except insects and mites.

Aschelminthes are pseudocoelomate, unsegmented worms that are covered with a cuticle. An unusual aschelminth trait is parthenogenesis (development of an unfertilised egg). In the classes Nematoda, Rotifera and Gastrotricha, males may be lacking and successive generations of females are produced parthenogenetically

Still another remarkable characteristic of the phylum is the consistency of cell numbers (eutely), in which a precise and relatively small number of cells, remaining unvaried throughout the animal’s life, comprise both specific organs and the entire animal. The number, constant not only for the species but also for the taxonomic group, can be used as part of its morphological definition (for example, the central nervous system in Ascaris and many other nematodes consists of 162 cells)

Class Nematoda

The class Nematoda is very important to us among the group as it consists of parasites of medical and veterinary importance. The class is characterized by the presence of a complete digestive system, consisting of a mouth, intestine, anus and absence of cilia.

Nematodes are found practically in every ecological setting. They live in animals and plants as parasites and in mud, marine, fresh and brackish waters, soil and mud as free-living organisms. They vary in size from a few microns to slightly over a meter in length.

Morphology

Nematodes are cylindrical in shape, tapering at both ends of the body. The digestive system is a long tube that runs from the buccal cavity to the anus. In hookworms, the mouth is armed with teeth or cutting plates. Some nematodes are provided with lips, papillae or a leaf crown around the external opening of the mouth. The pharynx (oesophagus) is a strong muscular tube with walls that can contract and expand, creating a pump-like suction mechanism. The neuromuscular system consists of a circumesophageal ring or nerve ring around the oesophagus and dorsal and ventral nerve cords, which give rise to motor and sensory nerves. The excretory organs lie within the lateral lines and open via a pore ventral to the oesophagus.

                                           Internal structure of a nematode

The gonads lie in the pseudocoele, a space between the intestine and the body wall that is filled with a fluid. The rugged cuticle is not only an extraordinary effective protection against harmful external substances but also serves as an exoskeleton to which muscles are attached.

Female nematodes have a pair of ovaries consisting of cells (oogonia) that produce the eggs. The uteri are usually packed with eggs. The eggshell material is produced by the uterine cells. The uteri unite to form the vagina, which may contain a seminal vesicle for storing male sperm. The vagina opens to the outside by a pore or vulva at the middle of the body, though it may also be near either end of the body.

The male genital system consists of one testis and a seminal vesicle, which continues posteriorly as a muscular ejaculatory duct that opens into the rectum or cloaca. The cloaca serves as a conduit for sperm and digestive wastes. Copulatory spicules protrude through the cloaca opening. The spicules, usually two in number, unite and form a tube through which spermatozoa are injected into the vulva and vagina of the female.

Generalized structure of male (A) and female (B) nematode

Nematodes are classified into two subclasses: Phasmidia (have caudal sensory organs) and Aphasmidia (lack caudal sensory organs). The phasmids include most soil nematodes, most parasites of insects and vertebrates. The aphasmids are mainly aquatic forms and a few parasitic ones.

Life cycle

Life cycles differ greatly among the many nematodes that are parasites of man. Mature female worms produce eggs, which pass out of the body with faeces. Most of the eggs are produced when they are not embryonated and become embryonated in the soil. The embryonated eggs are ingested and under the action of the gastric juices of the host, the larvae are liberated from the eggs. In some parasites, such as Ascaris, the larvae penetrate the wall of the small intestine and migrate to the lungs via the blood. From the lungs, they reach the pharynx and finally settle in the small intestine, where they will attain maturity, mate and produce eggs.

In other nematodes, such as hookworms, the larvae hatch from eggs in the soil and after going through several moults become infective. These third stage infective forms enter the body through the skin. Trichinella has a unique life cycle in nematodes in that the entire life cycle is spent in the host. Transmission from one host to another is through predation, cannibalism or carrion feeding.